What is Knee on a Compressor?

On the road, doing some session work. I got a little break here during a session, so I want to tell you guys about a compressor setting called the “knee” function. I’m going to explain to you what that is and why you would use it.

So I’ve got this lead vocal here, and I’ve got a compressor on it. Let’s check that out.

[mix]

So you see that right now, the way it’s set, whenever the vocal gets to a certain level of dynamic, like a certain loudness, then it kicks the compressor in and the compressor starts working. That’s called a hard knee. It means when you’re above the threshold of amplitude that it allows, that’s when the compressor is working, and as long as you’re below that, then the compressor is not going to do any work at all.

Now, you can see here on the graph, it looks like a totally linear line here, and then it just shelves off. This is the demonstration of the compressor action.

Now what I’m going to do is soften the knee, and this is saying how far below the threshold does the compressor kick on, and if I set it to like, 20 decibels, that means that the compressor is actually going to start doing subtle amounts of gain reduction well below where the actual threshold is. So now I’m going to play it again.

[mix plays]

So you can see this time around that the compressor is acting on the quieter signals as well, and while it’s now sort of too much, right? It’s pulling the quiet signals down too effectively, but let’s split the difference. Let’s go for maybe about 10 dB. And when you’re setting a knee, it doesn’t have to be exact.

Let’s give that a listen.

[mix playback]

That sounds like a fairly decent balance.

ADVERTISEMENT

Now what I’m listening for is the transparency of the compressor action.

The reason why you might want to play with the knee control a little bit is because it makes the transition from not compression to compression subtler. So, while it makes the tone and action of the compressor a bit more homogenous over the overall signal, because it’s doing that, it also means that when the compressor is really kicking in, it’s not quite as obvious.

So one spot that you very commonly want to experiment with the knee is on something that’s meant to sound homogenous and is also very dynamic. So something like vocals, or piano, or acoustic guitars that have very dramatic differences in volume in certain sections.

Those are all places where you might want to work with the knee.

Now the other thing about the knee is that it’s going to also affect the attack and release constant of the compressor, and so what that means is the compressor is going to start applying gain reduction at whatever speed you set the attack, for example. It’s going to do that based on where the compression action begins.

So when you have a hard knee, then your compression is kicking in only when the threshold has been breached, which is at louder parts of the signal.

If you have a softer knee, which is what we have now, then it’s going to start kicking in well below the threshold, and therefore, the attack constant is effectively going to be speeding up in a way.

So what did that all mean? Well, it means ultimately that you can get a smoother and more rounding attack effect, and that can be useful if you’re trying to really soften a snare drum, for example. Rather than just simply use a quick attack, what you might want to do is use a quick attack and also soften the knee.

Now, the other thing to realize about this knee setting is that this compressor that I’m demonstrating it on has a variable knee. Not every compressor has a variable knee, but every compressor has it’s own knee action, and so when you have things like the SSL buss compressor for example, you might find that the knee of the action is softer at 2:1. It’s a little bit harder at 4:1, and it’s almost a hard knee completely at 10:1.

So this is how getting to know and understand your compressor can be really helpful in terms of determining what compressor is best for what source.

Alright guys, so that’s the knee on a compressor. I hope that you learned something. Until next time.

Matthew Weiss

Matthew Weiss is the recordist and mixer for multi-platinum artist Akon, and boasts a Grammy nomination for Jazz & Spellemann Award for Best Rock album. Matthew has mixed for a host of star musicians including Akon, SisQo, Ozuna, Sonny Digital, Uri Caine, Dizzee Rascal, Arrested Development and 9th Wonder. Get in touch: Weiss-Sound.com